Optical performance monitoring is becoming increasingly important, particularly in dense wavelength division multiplexed (DWDM) optical communication systems. The main drivers for signal monitoring are to identify changes in transmitted signals, to diagnose the cause and location of the underlying faults, to optimize the performance of tunable elements, and to estimate the bit-error rate (BER).
Currently, there are two classes of optical signal monitors, namely frequency domain monitors and time domain monitors. Frequency domain methods analyse the spectral content of the signal. These methods average the signal over time and therefore contain little or no information about signal distortion. The optical spectrum analyser is an example of a device for frequency domain monitoring.
Time domain signal monitoring techniques sample the waveform, whether asynchronously or synchronously. Such time domain techniques are sensitive to signal distortion and noise.
Synchronous time domain techniques require a clock to be extracted from the signal so that the sampling can be synchronized to the signal bit rate. Such techniques include sampling oscilloscopes which produce eye diagrams (being a plot of sample amplitude vs. time), and Q-factor monitors. The synchronous waveform monitor (also known as a digital sampling oscilloscope, whether real time or interleaved) is a test and measurement device which measures the eye pattern of the optical waveform. However, the synchronous technique relies on access to the signal clock to align sampling times with the bit sequence. In the laboratory setting the clock is readily available as one has access to the data source. In the field the clock has to be recovered from the data, using a clock extraction circuit. Not only does clock extraction involve added expense, a typical clock extraction circuit only works over a limited range of bit rates and formats. Another type of synchronous sampling technique is described in U.S. Pat. No. 6,904,237, in which a histogram of sample density against sample amplitude is produced.
In contrast, asynchronous sampling techniques such as asynchronous histograms do not require clock extraction circuitry and are therefore cheaper to implement and are transparent to bit rate. An asynchronous time domain sampling technique is described in U.S. Pat. No. 6,836,620, in which a histogram of sample density against sample amplitude is produced.
The shape of such asynchronous and synchronous histograms changes as the signal becomes degraded, and significant effort has been put into correlating these changes with various degradation mechanisms. However, differentiating between degradation mechanisms is difficult, particularly when they occur simultaneously, because different degradation mechanisms can cause similar changes in histogram shape.
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